Electrostatic printing



Aug. 24, 1965 c. o` cHlLDREss 3,202,093

ELCTROSTATIC PRINTING Filed Feb. 11, 1963 RECIPROCAL 2| ACTUATOR 24 lll' -J 22 j RECIPRQCAL 2\ 26 ACTUATOR 50 32 INVENTOR. CLYDE O. @HALO/@55 mja/Mw@ United States Patent O 3,202,093 ELECTROSTAT C PRINTN Clyde 0. Childress, Palo Alto, Calif., assigner to Electrostatic Printing Corporation of America, San Francisco, Calif., a corporation of California Filed Feb. 11,1963, Ser. No. 257,614 Claims. (Cl. lOl-114) powder particles are transferred by an electric field, to-

ward a conductive object which is opposite the screen and spaced therefrom. A potential is established between the conductive objects and the screen to set up the electric field thereacross for directing the powder particles in the pattern established by the screen apertures toward the conductive object; Printing may occur on the conductive object or upon an object interposed between the conductive object and the screen.

The powder particles are entered into the electric field by means of a brush which is used to brush these particles through the screen apertures defining the desired pattern. In an application for Selective Printing Using Electrostatic Techniques, bearing Serial No. 246,139, filed December 20, 1962, by Clyde O. Childress and assignedY to this assignee, there is described a technique whereby the brush is eliminated and instead carrier particles are mixed with the powder particles. are larger than the screen apertures while powder particles are smaller than the screen apertures. When the' container for the particles, with the screen at one end, is brought rapidly toward the other electrode used to establish the electric field and then the motion of the container is abruptly arrested, the carrier particles act to Vforce powder through the screen apertures into the field to effectuate the printing operation. vIn another application for an Electrostatic Printing System by this inventor, Serial No. 244,150, filed February 41, 1963, now abandoned the carrier particles are made magnetic. As a result, by establishing a magnetic field from the printingv side of the screen the magnetic particles are pulled toward the screen and in doing this they literally rub the powder particles through the screen apertures thereby entering them into the electric field to be carried toward the opposite electrode in that field.

Heretofore, for the purpose of establishing the electric field for transferring particles through this screen toward the opposite electrode, the screen has been made conductive, for the purpose of acting as one of the electrodes in establishing the electric field. The reason for using the Screen is that it is a convenient mechanism for establishing a field having the desired area and because of its proximity to the opposite electrode the voltage requirement for establishing a field having the required transfer' capabilities are not as high as would occur if the electrode which establishes the field were for example, at the top of the container holding the powder particles. A phenomenon which occurs with certain electroscopic powders particularly when printing directly onto conductive surfaces y The carrier particles Multiple powder l particle passage of this type between electrodes can cause either a displacement of the location where the powder is supposed to be deposited on theA material on-which writing is supposed to occur, or can cause some overalll nonuniformity of the image pattern which is deposited. Accordingly it is desirable to eliminate the effect described, namely, powder return to the screen, or multiple passage of the power.l Some further difficulties experi-l ence'd with conductive screens is that it is relatively difficult ,to keep them taut and without creases whereas monolament screens of nylon or other materials are'l easily stretched andmaintained taut.

Accordingly, an object of this invention is to provide an electrostatic printing process in which effects such as powder return and multiple powder passage are minimized. i

Another object of this invention is to provide an improved electrostatic printing process Wherein the required field is established without using the screenas an elec` trode for this purpose.

Yet another object of the present invention is the provi-y sion of a novel arrangement for establishing an electric fieldin an electrostatic printing process.

Yet another object of the present invention is to pro-Y vide an electrostatic printing system in which the required field is effectuated although the screenis made s nonconductive.

Still another object of this invention is to provide an improved novel and useful electrostatic printing process. These and other objects of thisk invention are achieved in an arrangement wherein there is mixed with the powf der particles carrier particles which are conductive. The

mixture is held in a container, one side of which cornprises a screen with apertures arranged in a desired printing pattern. The` screen is not made of conductive material. so that they constitute a layer over the apertured regions of the screen, and so that they literally push the adhering powder particles through the openings in the screen. The

electric field required is established by connecting the that by providing a mixture in which aproximately50% s or more of the mixture by volume is constituted of the carrier particles, then the field which is set up by connecting these carrier particles4 to the potential source is sufficiently uniform to permit printing of excellent quality to occur. Since the screen'is now made nonconductive the multiple passage phenomenon of the powder particles is substantially eliminated. The particles do not reverse their charge upon reaching the non-'conductive screen and will therefore remain ,at the screen. f

The novel features that are considered characteristic of this invention are set forth with particularity in the appended claims. The invention itself, both as to its organization and method of operation, as Well as addi' tional objects and advantages thereof, will best be understood from the following description when read in conjunction with the accompanying drawings, in which: f

FIGURE l is a diagram showing the disposition in a container of a mixture of triboelectric powder and conductive powder particles; v

FIGURE 2 is a perspective View of an embodiment of the invention;

particles Means are provided for moving the carrier particles FIGURE 3 is a perspective view of another embodiment of the invention;

FIGURE 4 shows the details of a container which may be employed in the embodiment of the invention shown in FIGURE 3, and

FIGURE 5 is a drawing of another embodiment of the invention.

Reference is now made to FIGURE 1 which shows the appearance of a container il@ with one wall shown partially open, having a mixture of carrier particles 12 and powder particles M therein, the screen is at the bottom of the container. With a sufficient quantity of carrier particles 12 mixed with the powder particles I4, (preferably 50% by volume) a probe or an electrode which is inserted into the mixture at any location can serve to connect substantially all of the carrier particles to a source of potential. The conductive carrier particles should be sized large enough not to pass through the screen aperture, while the powder particles are small enough to pass through the screen apertures. Enough of the carrier particles touch one another in the mass of the mixture to establish a conductive layer extending through the mass. The carrier particles should not be so small that they fit into the screen apertures. As an example, a 200 mesh screen was used for printing. The conductive particles used were those which passed through a 100 mesh screen but not through a 150 mesh screen.

Reference is now made to FIGURE 2 which is an isometric view of an embodiment of the invention. A container 2t) is reciprocally actuated by any suitable reciprocal actuator 21. The actuator may be a reciprocally driven solenoid or a bell crank device which is driven from a motor. The container 2G has a working face 22 which is made of nonconducting material. The working face includes screen apertures 22A which dene the desired printing pattern. Supported in the working face 22 are contacts 24 which serve the purpose of enabling a source of potential to be connected to the contents of the container 2t). These contents comprise a mixture of conductive carrier particles with the powder particles desired to be transferred through the screen apertures to the material 26 on which printing is to occur. Mixed with the powder particles are carrier particles such as 14 in FIGURE l, which are conductive. These carrier particles may be made, for example, from any conductive metal such as iron, or aluminum, or any conductively coated material, such as glass spheres with a conductive coating thereon. rThe powder particles are sufcicntly small to pass through the screen apertures whereas the carrier particles are not.

A source of potential 23 has one terminal connected to two arresting contacts 30, 32, and the other terminal connected to the material 26 upon which printing is to occur. Preferably, the surfaces of the arresting contacts 30, 32, which face the container 2t? are made of conductive material. rThe container is moved until it touches the arresting contacts 30, 32. At this time a connection is made between the conductive carrier particles in the container, through the contacts 2li, through the arresting contacts 30, 32. As a result, an electric icld is established between the conductive carrier particles and the conductive material 26 whereby a powder particles which are urged through the screen apertures into the electric eld are transferred to the 'material 26. The reciprocal actuator 21 moves the container 2t) rapidly until it irnpacts the contacts 30, 32, and then rapidly withdraws the container. A new conductive surface can then be substituted for the one presented by the conductive material 26.

An arrested impact type of Operation such as is shown in FIGURE 2 need not be employed. An arrangement may be used such as shown in FiGURE 3, wherein the container 34 has a nonconducting working surface 36 with the screen apertures 36A arranged in a pattern which it is desired to print. The inside of the container 3f.- is shown in FIGURE 4. it consists of a series of rods or wires 33, which are disposed in any desired pattern through the container. These rods connect to the outside of the container to at least one terminal 40. Connection is made between this one terminal and the source of potential 42 shown in FIGURE 3. The container 34 holds a mixture of tribo-electric powder particles and conductive carrier particles. These conductive carrier particles under the iniiuence of the reciprocal actuator 21 are moved toward the back of the container when the reciprocal actuator pulls the container toward itself and then suddenly reversed. When the reciprocal actuator moves the container toward the conductive material 26 upon which printing is to occur and then suddenly again reverses the motion of the container the carrier particles are thrust against the nonconductive screen apertures pushing the powder particies through these apertures and by virtue of the electric field which is established the powder particles travel toward the material 26.

If desired, the container walls may be made conductive and with small area printers this provides a suiiicient contact for enabling the conductive particles to act as a eld electrode.

Since the printing process takes an interval on the order of a millisecond for transferring the power particles from the screen to the conductive material 26, the reciprocal actuator can move the container at a fairly high speed without actually interfering with the printing process. It will be understood that in accordance with the basic description of this process, if it is desired to print on materials which are not connected to the source of potential 42, then these materials may be interposed between conductive material 26 and the screeen 36 for the purpose of being printed on.

FIGURE 5 shows an arrangement for using conductive magnetic carrier particles to establish an electrode for elfectuating electrostatic println". The container 5t) holds the mixture of conductive carrier particles, which are capable of being magnetized, and the pigment powder particles which it is desired to transfer into the electric eld region. The electric field region is the region between a conductive electrode 52 and the electrode established by the conductive carrier particles. The surface on the container Si) is the working surface in which there are screen apertures arranged in a desired printing pattern. The screen is made of nonconductive material. A document or an object on which it is desired to be printed may be inserted between the electrode 52 and the working face 542-. A source of potential 56 is connected between the electrode 52 and contacts 58, titi, which extend a distance into the container 5t) to make contact with the conductive carrier particles.

A source of potential 62 is connected oy means of a switch 6ft to an electromagnet 66, consisting of the yoke 65 and the tapped winding 70 thereon. A selector switch '72 is used to sequentially connect the taps of the winding 7) to the source of potential 62.

When it is desired to print, the switch 64 is closed, whereby the left end of the electromagnet 6o provides a magnetic field which attracts the conductive magnetic particles toward the screen surface 54. These particles come in contact with the terminal S0 when they are pulled up from the bottom of the container toward the surface 54. The conductive magnetic carrier particles move the powder particles through the screen apertures into the electeric cld which is established by virtue of these conductive carrier particles making contact with the terminal 6i). The switch 72 is moved to successively engage the taps on the coil of thc electromagnet wherby the conductive carrier particles are successively drawn against the operating surface 54 of the container 5t) to thereby both move the powder particles into the region between the screen and the conductive plate 52 and to also establish an electric held. At the end of the printing operation the switch may be opened whereby the conductive carrier particles drop back into the powder mass to thereby renew the powder charge which will be used for the succeeding Writing operation.

By using the conductive carrier particles as the electrodes for electrostatic printing rather than the screen, one is enabled to make the screens out of less expensive materials than heretofore. The requirement that the screen be made conductive necessitated either an expensive metal screen or one that could be coated with a conductive coating. The screen therefore can now be made of much cheaper materials. Thus, is addition to improving the resultant electrostatic printing which is achieved by the techniques described herein, by this invention the cost of the apparatus is also made less.

There has been described herein above, a novel, useful improvement in electrostatic printing.

` I claim:

1. In an electrostatic printing system of the type having two spaced electrodes for establishing an electric eld therebetween, one of said electrodes being a screen having apertures formed in a desired pattern and having electroscopic powder particles inserted into said electric eld in said desired pattern through the apertures of said screen from a container of said powder particles adjacent said screen, the improvement comprising a plurality of conductive carrier particles mixed with said powder particles within said container, said conductive carrier particles being larger in size than the apertures in said screen and said powder particles being smaller in size than the apertures in said screen, means for urging said conductive carrier particles toward the screen for enabling them to move said powder particles through the apertures of said screen into said electric eld, and means for applying a potential between said conductive carrier particles within said container and said other spaced electrode to establish said conductive carrier particles as an electrode for forming said electric leld.

2. The improvement as recited in claim 1 wherein said conductive carrier particles constitute at least fty percent by volume of said mixture of carrier and powder particles.

3. An electrostatic printing system comprising a hollow container having as the base thereof a substantially nonconductive screen having apertures therethrough arranged in a desired printing pattern, a conductive medium positioned spaced from said screen and adjacent one side thereof, a mixture of powder particles and conductive carrier particles within said container, the size of said powder particles being smaller than said screen apertures to pass therethrough, the size of said carrier particles being larger than said screen apertures not to pass therethrough, means for moving said container for urging said conductive carrier particles toward the other side of said screen for enabling said conductive carrier particles to push said powder particles through said screen apertures, a source of potential electrode means positioned in said container for contacting said mixture of powder particles and carrier particles, and means including said electrode means for connecting said source of potential between said conductive carrier particles and said conductive medium for establishing an electric eld therebetween to cause said powder particles to transfer to said conductive medium.

4. An electrostatic printing system comprising a hollow container having as the base thereof a relatively nonconductive screen having apertures therethrough arranged in a desired printing pattern, a relatively conductive medium positioned spaced from said screen and adjacent one side thereof, a mixture of powder particles and conductive carrier particles within said container, the size of said powder particles being smaller than said screen apertures to pass therethrough, the size of said carrier particles being larger than said screen apertures not to pass therethrough, means for moving said container for urging said conductive carrier particles toward t-he other side of said screen for enabling said conductive carrier particles to push said powder particles through said screen apertures, a source of potential, and means including a wall of said container for connecting said source of potential between said conductive carrier particles and said conductive medium for establishing an electric tield therebetween to cause said powder particles to transfer to said conductive medium.

5. An electrostatic printing system as recited in claim 4 wherein said conductive carrier particles constitute at least fty percent by volume of said mixture of carrier and powder particles.

References Cited by the Examiner UNITED STATES PATENTS 2,874,063 2/59 Greig l17--l7.5 2,930,351 3/60 Giaimo 118-637 2,956,487 10/ 60 Giaimo.

2,966,429 12/ 60 Darrel et al.

3,037,478 6/62 Lace 117-175 3,081,698 3/ 63 Childress.

ROBERT E. PULFREY, Primary Examiner. DAVID KLEIN, WILLIAM B. PENN, Examiners. 

4. AN ELECTROSTATIC PRINTING SYSTEM COMPRISING A HOLLOW CONTAINER HAVING AS THE BASE THEREOF A RELATIVELY NONCONDUCTIVE SCREEN HAVING APERTURES THERETHROUGH ARRANGED IN A DESIRED PRINTING PATTERN, A RELATIVELY CONDUCTIVE MEDIUM POSITIONED SPACED FROM SAID SCREEN AND ADJACENT ONE SIDE THEREOF, A MIXTURE OF POWDER PARTICLES AND CONDUCTIVE CARRIER PARTICLES WITHIN SAID CONTAINER, THE SIZE OF SAID POWDER PARTICLES BEING SMALLER THAN SAID SCREEN APERTURES TO PASS THERETHROUGH, THE SIZE OF SAID CARRIER PARTICLES BEING LARGER THAN SAID SCREEN APERTURES NOT TO PASS THERETHROUGH, MEANS FOR MOVING SAID CONTAINER FOR URGING SAID CONDUCTIVE CARRIER PARTICLES TOWARD THE OTHER SIDE OF SAID SCREEN FOR ENABLING SAID CONDUCTIVE CARRIER PARTICLES TO PUSH SAID POWDER PARTICLES THROUGH SAID SCREEN APERTURES, A SOURCE OF POTENTIAL, AND MEANS INCLUDING A WALL OF SAID CONTAINER FOR CONNECTING SAID SOURCE OF POTENTIAL BETWEEN SAID CONDUCTIVE CARRIER PARTICLES AND SAID CONDUCTIVE MEDIUM FOR ESTABLISHING AN ELECTRIC FIELD THEREBETWEEN TO CAUSE SAID POWDER PARTICLES TO TRANSFER TO SAID CONDUCTIVE MEDIUM. 